South Stream is one of several competing pipeline projects launched in recent years intended to deliver gas from Russia, the Caspian Sea and beyond to southern Europe which is seeking to diversify its gas supply.

It is being built by Russian energy giant Gazprom, with Italy's ENI, French firm EDF and Germany's Wintershall for an estimated 16.5 billion euros, though some experts feel it will cost much more.

South Stream will compete with the European Union supported Nabucco which has been repeatedly postponed and which is now considering a shorter and less expensive route.

There is also the proposed TGI - Turkey, Greece, Italy - pipeline and the Transadriatic Pipeline or TAP.

Decisions on which will get the final go-ahead are likely next year.

South Stream's planned capacity is 63 billion cubic metres per year and Gazprom says the first gas deliveries should flow through the pipeline by the end of 2015.

All this comes against a background of the EU launching a competition investigation into Gazprom's gas flows and prices.

Moscow and Brussels are also at odds over Europe's pipeline access rules, which Gazprom has complained hinders investment in one of its major markets and creates uncertainty for the South Stream project itself.